There has been an evolutionary development in tissue stimulating apparatuses. The advent of programmable tissue stimulators has allowed for the optimal pacing system for a specific individual, simplified troubleshooting of stimulator problems, and noninvasive changes in the stimulator programming.
At present, the newest and most sophisticated mode programmable cardiac pacing apparatuses (which are a subset of tissue stimulators) are designed to pace in the DDD mode. These apparatuses, while having the ability to stimulate (pace in the DDD mode) can also be programmed to pace in the AOO, VOO, VVI, AAI, AAT, VVT, VDD, DOO and DVI. The DDD mode of operation, at present, is the state of the art mode in pacing.
The DDD mode possesses the characteristics of truer physiologic pacing because of the advantages in its hemodynamic and electrophysiologic abilities.
The DDD mode of operation is designed to mimic the cardiac cycle electronically. Therefore, atrial or ventricular stimulation alone or atrial and ventricular stimulation in sequence will be delivered, so as to continuously maintain atrial and ventricular synchrony over a wide range of rates.
When a cardiac pacing apparatus is operated in the DDD mode, stimulation is provided to: (1) the atrium only in the presence of an atrial bradycardia with intact A-V conduction, (2) the ventricle alone in the presence of normal sinus rhythm in the absence of A-V conduction, and (3) both the atrial and ventricle in the presence of bradycardia in both chambers.
When the cardiac pacing apparatus is operated in the DDD mode, ventricular sensing is inhibited for a period of time after atrial stimulation to prevent cross-talk. This refractory or blanking period prevents the apparatus from sensing in the ventricle and falsely interpreting an atrial output pulse as a true ventricular depolarization. However, a drawback is that ventricular activity occurring during this blanking period may go undetected.
The DDD mode of operation for a cardiac pacing apparatus has been found to be ineffective in situations in which there is an electrically unstable atrium as evidenced by intermittent atrial flutter/fibrillation or frequent extra-systoles, or slow retrograde atrial activation which triggers ventricular pacing. DDD mode pacing is ineffective in these situations because the atrium cannot be stimulated, or atrial depolarization cannot be consistently sensed, or the timing of the atrial signal is inappropriate for governing physiological ventricular activation.
To combat many of the problems associated with pacing in the DDD mode only, the DDD mode pacing apparatus can reprogram from among the currently known pacing modalities, i.e., AAI, VVI, VDD, or DVI, in order to approximate normal physiological cardiac functions in the presence of bradycardia rhythm disturbances.
In recent years, some of these same pacing apparatuses have been equipped to cope with tachycardias. The techniques incorporated in these apparatuses to terminate tachycardias were either competitive underdrive pacing (asynchronous competitive pacing below the rate of the tachycardia); or burst or overdrive pacing (a short burst of rapid stimulation at a rate faster than the tachycardia); or delivery of programmed stimuli (emission of a single or double (or more) stimuli at a precise time to break the tachycardia). Such tachycardia terminating pacing apparatuses had external activation in early systems which dictated the method which is activated to terminate the tachycardia. Later there was the devleopment of automatic systems which would automatically provide the programmed method for breaking a tachycardia.
The present day DDD mode cardiac pacing apparatus, although approaching the point of being a true physiological pacing apparatus because of the conjunctive use of the other programmed pacing modes incorporated therein, still has many problems. These problems are associated with arrhythmias caused by pacing in a programmed DVI mode where there is no sensing in the atrium, or noneffective atrial stimulation which shortens battery life. There are also problems when pacing in the DDD mode when an R-wave that is premature (Premature Ventricular Contraction, PVC or junctional beat) is sensed and is followed by a slow retrograde atrial activity. This can result in a tachycardia consisting of a slow V-A retrograde pathway in the heart followed by sensing of that retrograde P-wave by the pacemaker and subsequent pacing of the ventricle, which is known as "pacer mediated tachycardia".
Besides particular mode problems associated with the present day DDD mode pacers, there are also problems in evaluating the performance of implanted DDD pacers due to their complexity when viewed on a surface ECG apparatus.
Besides those problems associated particularly with DDD mode pacing apparatus, there are other general problems with programmable pacing apparatus. Consistently there are problems in determining if a signal sensed on the P-channel of the pacing apparatus is actually a P-wave or merely noise. The existence of a valid P-wave will cause the pacer to be inhibited, where noise will not. Since there is no proper method to validate whether what is detected on the P-channel is noise or a valid P-wave the pacer can improperly acknowledge noise as a P-wave or not acknowledge a P-wave because it thought it was noise.
The present invention solves these and other problems associated with programmable pacing apparatus capable of pacing in the DDD mode.